Diesel engine exhaust is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and nitrogen oxides (“NOx”), but also condensed phase materials, i.e. liquids and solids, which constitute the so-ca lied particulates or particulate matter. Emissions treatment systems for diesel engines must treat all of the components of the exhaust to meet the emission standards set by the various regulatory agencies throughout the world.
The total particulate matter emissions of diesel exhaust contain three main components. One component is the solid, dry carbonaceous fraction or soot fraction. This dry carbonaceous fraction contributes to the visible soot emissions commonly associated with diesel exhaust. A second component of the particulate matter is the soluble organic fraction (“SOF”). The SOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust. It is generally present as condensed liquids at the standard particulate collection temperature of 52° C. in diluted exhaust, as prescribed by a standard measurement test, such as the U.S. Heavy Duty Transient Federal Test Procedure. These liquids arise from two sources: (1) lubricating oil swept from the cylinder walls of the engine each time the pistons go up and down; and (2) unburned or partially burned diesel fuel. The third component of the particulate matter is the so-called sulfate fraction, which is formed from small quantities of sulfur components present in the diesel fuel.
Catalyst compositions and substrates on which the compositions are disposed are typically provided in diesel engine exhaust systems to convert certain or all of these exhaust components to innocuous components. For instance, oxidation catalysts that contain platinum group metals, base metals and combinations thereof, facilitate the treatment of diesel engine exhaust by promoting the conversion of both unburned hydrocarbons (HC) and carbon monoxide (CO) gaseous pollutants, and some proportion of the particulate matter through oxidation of these pollutants to carbon dioxide and water. Such catalysts have generally been disposed on various substrates (e.g. honeycomb flow through monolith substrates), which are placed in the exhaust of diesel engines to treat the exhaust before it vents to the atmosphere. Certain oxidation catalysts also promote the oxidation of NO to NO2.
In addition to the use of oxidation catalysts, diesel particulate filters are used to achieve high particulate matter reduction in diesel emissions treatment systems. Known filter structures that remove particulate matter from diesel exhaust include honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal filters, etc. However, ceramic wall flow filters, described below, receive the most attention. These filters are capable of removing over 99% of the particulate material from diesel exhaust. Typical ceramic wall flow filter substrates are composed of refractory materials such as cordierite or silicon-carbide. Wall flow substrates are particularly useful to filter particulate matter from diesel engine exhaust gases. A common construction is a multi-passage honeycomb structure having the ends of alternate passages on the inlet and outlet sides of the honeycomb structure plugged. This construction results in a checkerboard-type pattern on either end. Passages plugged on the inlet axial end are open on the outlet axial end. This permits the exhaust gas with the entrained particulate matter to enter the open inlet passages, flow through the porous internal walls and exit through the channels having open outlet axial ends. The particulate matter is thereby filtered on to the internal walls of the substrate. The gas pressure forces the exhaust gas through the porous structural walls into the channels closed at the upstream axial end and open at the downstream axial end. The accumulating particles will increase the back pressure from the filter on the engine. Thus, the accumulating particles have to be continuously or periodically burned out of the filter to maintain an acceptable back pressure.
Catalyst compositions deposited along the internal walls of the wall flow substrate assist in the regeneration of the filter substrates by promoting the combustion of the accumulated particulate matter. The combustion of the accumulated particulate matter restores acceptable back pressures within the exhaust system. These processes may be either passive or active regeneration processes. Both processes utilize an oxidant such as O2 or NO2 to combust the particulate matter.
Passive regeneration processes combust the particulate matter at temperatures within the normal operating range of the diesel exhaust system. Preferably, the oxidant used in the regeneration process is NO2 since the soot fraction combusts at much lower temperatures than those needed when O2 serves as the oxidant. While O2 is readily available from the atmosphere, NO2 can be actively generated through the use of upstream oxidation catalysts that oxidizes NO in the exhaust stream.
In spite of the presence of the catalyst compositions and provisions for using NO2 as the oxidant, active regeneration processes are generally needed to clear out the accumulated particulate matter, and restore acceptable back pressures within the filter. The soot fraction of the particulate matter generally requires temperatures in excess of 500° C. to burn under oxygen rich (lean) conditions, which are higher temperatures than those typically present in diesel exhaust. Active regeneration processes are normally initiated by altering the engine management to raise temperatures in front of the filter up to 570-630° C.
With increasing standards, also the emission of NOx is regulated, and modern Diesel engines have to fulfill certain requirements regarding the overall emission of NOx. The emission of NOx may be reduced by a “lean NOx trap” (LNT) catalyst, unusually applied in combination with a catalyzed soot filter (CSF), and the LNT is usually applied upstream of the CSF. However, also LNT catalysts require sulfur regeneration from time to time, which may be achieved by a sequence of short, rich (stoichiometric ratio lower than 1) pulses. During this operation, the LNT releases unwanted hydrogen sulfide (H2S), which has to be minimized. This function of an H2S reduction may also be applied to the CSF. But the CSF still has to perform other functions, such as a conversion of HC to CO, and the two functions influence each other. It is in particular known that a contact of the active components that are used as H2S suppression material and CO conversion material may reduce the efficiency of the catalytic function, especially of the CO conversion material.
It is an object of the present disclosure to provide a catalyzed soot filter (CSF) with a combined function of a H2S reduction and a CO reduction having a high H2S conversion rate and a reduced CO emission during normal engine operations and regeneration events, in particular active filter regeneration.
It is also an object of the present disclosure to provide a catalyzed soot filter (CSF) with a combined function of a H2S reduction and a CO and HC reduction having a high H2S conversion rate and a reduced CO and HC emission during normal engine operations and regeneration events, in particular active filter regeneration.
It is another object of the present disclosure to provide a catalyzed soot filter (CSF) with reduced back pressure.
It is still another object of the present disclosure to provide a catalyzed soot filter (CSF) with an increased CO conversion rate at a low H2S emission rate.